Accurate measurement of fluid flow rates in an environment are highly desirable in a wide variety of situations. The type and nature of available flow meters vary considerably and the choice of a particular flow meter will depend strongly on the intended application.
One type of flow meter is based on measuring the rotational velocity of a rotatable member when placed in the fluid to be measured. The passing fluid exerts a drag force onto fins or blades of the rotatable member, which rotates in response in a known manner.
A convenient and widely used method to measure the rotational velocity of the rotating member is to utilise a Hall effect sensor, involving locating a permanent magnet on the rotatable member. As the member rotates, the distance between the magnet and a circuit oscillates, producing a measurable effect with a frequency equal to that of the oscillation. This method is highly favoured because it does not require physical contact with the rotating member, giving a more accurate reading.
When operating an oilfield wellbore, whether during drilling or during production, accurate measurements of fluid flow rates is of great importance. In a recent development, devices which employ Hall effect sensors on a rotating flow meter have been successfully used in the field, giving accurate flow readings in the harsh environment of an underground wellbore.